Vehicular brake mechanisms generate braking torque to the vehicle's wheels, comprising, for example, drums with friction linings or a caliper that presses friction pads against disks. The braking mechanism is designed so that application of force will cause braking torque build-up, while release of the force will remove the braking torque. In motor vehicles, the service brake system generally consists of hydraulically operated disk or drum brakes actuated by foot force, normally vacuum or pressure boosted. The parking brake is normally manually activated on a pair of front or rear wheels by means of mechanical links boosting the hand force and performing the same disk clamping or drum shoe expanding as the hydraulic service system. Steel wire cables, partially running in flexible spring sleeves, are used to deliver the braking force from the hand brake lever to the associated wheels
In order to eliminate the long and complex running path of the wire cables, Electric Parking Brake (EPB) systems have been provided in vehicles. Such systems allow remote, and sometimes, automatic application and release of the parking brake. These systems typically include an electro-mechanical actuator connected to the service brake either by a cable, as in the drum configuration, or directly attached to the caliper. The actuator converts electrical current to rotational mechanical output power for pulling the cable or driving a screw and apply the brakes. Unlike traditional manual or pedal-operated parking brakes, the actuator is driven to activate the parking brake by an electrical switch ergonomically located on the dashboard panel. The parking brake may be automatically deactivated when shifting to drive gear. In addition to enhanced convenience and freed space, this system may prevent human errors such as forgetting to release the parking brake.
Various construction alternatives were proposed during the last years, examples of such prior arts, using cable tensioning mechanism, can be found in U.S. Pat. No. 7,021,415, U.S. Publications 2004/0026195, 2005/0000762, 2005/0225166 (to the same applicant), 2006/0157308, 2006/0169553, 2006/0231352. Examples of direct coupled devices can be found in U.S. Publications 2006/0169548, 2006/0267402.
In electrical parking brakes, the measured current consumption of the electric motor is used to determine the braking force. The current consumption of the motor raises gradually while braking force is applied, until a certain predefined current level is drawn indicating that the desired braking force has been reached. A current cutoff is activated to stop the motor at the desired braking force. A self-locking worm gearing or small pitch lead screw drive are typically used to maintain the applied force despite cutting of the electric motor.
The use of such low efficiency self-locking drives, however, require the use of bigger motors having higher current consumption and lower sensitivity in determining the desired braking force. Furthermore, a longer period of time may be required to apply or release the brake. Other prior art electromechanical parking brake systems facilitate transition from the circular motion of the motor to the linear motion of the cable by utilizing a reeling mechanism or serrated rack. These methods feature limited longevity.